Photovoltaic Array Grounding Resistance Testing: Standards, Methods, and Acceptance Criteria

Photovoltaic array grounding resistance testing verifies that the earthing and bonding systems of solar power plants can safely dissipate fault currents and lightning surges into the soil. This technical guide covers the applicable IEC, IEEE, and GB standards, typical test points, industry-recognized measurement methods, and common acceptance criteria, helping EPC contractors, plant owners, and O&M teams ensure electrical safety and regulatory compliance throughout the project lifecycle.

Technical Overview

Grounding (earthing) is a fundamental safety requirement for every photovoltaic installation. A PV array typically spans thousands of square meters of metallic frames, rails, and enclosures, often installed in open terrain with high lightning exposure. An effective grounding system performs three critical functions: it provides a low-impedance return path that allows protective devices to clear faults quickly, it protects personnel from dangerous touch and step voltages, and it disperses lightning and switching surge energy into the earth before it can damage modules, inverters, or communication equipment.

Grounding resistance testing quantifies the resistance between the earthing system and remote earth, expressed in ohms. Because soil resistivity, electrode condition, and connection quality all degrade or vary over time, this testing is mandatory at commissioning and is repeated periodically during operation. IEC 62446-1 explicitly requires verification of protective bonding continuity and, where earth electrodes are installed, measurement of earth electrode resistance as part of grid-connected PV system acceptance testing. Accredited third-party laboratories and qualified O&M teams typically perform these tests using calibrated earth resistance testers compliant with IEC 61557-5.

Applicable Standards and Reference Documents

• IEC 62446-1: Grid-connected PV systems — minimum documentation, commissioning tests, and inspection requirements, including protective earthing verification
• IEC 62305 series: Lightning protection of structures, including design and testing of earth-termination systems
• IEC 62548: Design requirements for photovoltaic arrays, covering bonding and earthing provisions
• IEC 60364-7-712: Electrical installations of buildings — special requirements for photovoltaic systems
• IEC 61557-5: Performance requirements for measuring equipment used to test resistance to earth
• GB 50797: Code for design of photovoltaic power stations, specifying shared earthing grid requirements
• GB/T 32512 and GB/T 21431: Chinese standards for lightning protection and periodic inspection of PV power stations
• IEEE 80 and IEEE 81: Grounding of AC substations and guide for measuring earth resistivity and ground impedance
• ASTM G57: Wenner four-electrode method for field measurement of soil resistivity
• UL 2703 and NEC Article 690: Bonding of racking systems and PV-specific grounding rules used in North America

Testing Samples

PV Module Frames and Mounting Structures

Module frames, clamps, rails, and piers form the largest conductive surface of an array. Testing verifies that every metallic section is bonded to the grounding conductor and that the total path from any frame back to the earthing electrode meets the specified resistance limits. This is especially important for tracker systems, where rotating bearings can interrupt bonding continuity.

Inverters, Combiner Boxes, and Distribution Cabinets

The enclosures of string inverters, central inverters, DC combiner boxes, and AC distribution panels are sampled to confirm reliable connection to the protective earth conductor and the station earthing grid.

Lightning Protection Systems

Air terminals, down conductors, and their earth-termination electrodes are tested to confirm that the lightning protection earthing path remains intact and within the specified resistance threshold.

Grounding Grids, Electrodes, and Joints

Driven rods, horizontal buried conductors, exothermic welds, compression connectors, and inspection wells are examined and measured to detect corrosion, loosening, or theft of metallic conductors.

Ancillary Metallic Infrastructure

Transformer neutrals, cable trays, metallic conduits, perimeter fences, and station buildings are included in the sampling plan because they can introduce hazardous potential differences if left unbonded.

Testing Items

• Earth termination resistance of grounding electrodes and grids, measured in ohms
• Protective bonding continuity between module frames, racking, and equipment enclosures, typically measured in milliohms or low ohms with a micro-ohmmeter
• Soil resistivity at representative locations, expressed in ohm-meters, for design verification and corrosion risk assessment
• Mechanical and visual integrity of joints: torque verification, weld quality, corrosion, and conductor cross-section adequacy
• Verification that no unintended parallel earth paths distort the measurement result

Testing Methods

Fall-of-Potential (Three-Pole) Method

This is the reference method for commissioning and periodic inspection. A test current is injected between the electrode under test and a remote current electrode, typically placed 30 to 50 meters away or at least five times the maximum grid dimension. A potential probe is moved along the line between them, and the true earth resistance is read where the potential gradient plateaus, commonly near 62 percent of the current electrode distance. Repeating the measurement with the potential probe shifted helps confirm a valid, flat response curve.

Clamp-On (Stakeless) Method

A clamp-on earth tester induces a test current in the grounding conductor and measures the resulting loop resistance, provided a parallel return path exists through the interconnected grid. It is fast, requires no auxiliary electrodes or de-energization, and is well suited to routine O&M screening of individual electrode branches. A sharply elevated reading usually indicates a broken or corroded bond that must be investigated further.

Two-Point (Series) Method

Used for isolated electrodes or tight rooftop sites where driving probes is impractical, this method measures the series resistance of the electrode under test and a known-good reference ground, such as a metallic water system or the building main earthing terminal. It is approximate but adequate as a screening check.

Wenner Four-Electrode Soil Resistivity Method

Following ASTM G57 and IEEE 81, four equally spaced electrodes are driven to shallow depth and the apparent resistivity is calculated from the measured resistance and spacing. The results inform earthing design validation, seasonal variation studies, and cathodic-corrosion risk assessment.

Continuity Testing per IEC 62446-1

Using a low-resistance ohmmeter with a test current of at least 0.2 A, the resistance between accessible conductive parts and the protective earthing terminal is measured and compared against the value expected for the conductor length and cross-section. Consistently low, stable readings across sampled frames confirm a robust bonding network.

Typical Acceptance Criteria

Acceptance thresholds depend on the governing code, system design, and soil conditions, so the project specification and local authority having jurisdiction always take precedence. Commonly applied limits include values not exceeding 10 ohms for dedicated lightning protection earth terminations in many national codes, 4 ohms or less for shared protective and lightning earthing grids of PV power stations under Chinese design practice, and up to 25 ohms permitted by the NEC for a single grounding electrode unless supplemented. Large plants with collector substations may adopt 1 ohm or lower based on step and touch voltage studies. Test reports should record the measured value, method, probe spacing, soil and weather conditions, instrument model, and calibration status.

Applications

• Commissioning and acceptance testing of utility-scale ground-mounted solar farms before energization
• Verification of bonding and earthing on commercial, industrial, and residential rooftop systems
• Specialized installations such as floating PV, agrivoltaics, and solar carports, where earthing arrangements differ from conventional racking
• Scheduled O&M inspections, typically annual or semi-annual, aligned with lightning season preparation
• Post-event forensic assessment following lightning strikes, floods, or construction work near buried conductors
• Independent verification by accredited third-party laboratories for lender due diligence, insurance, and asset transfer

FAQ

What is the acceptable grounding resistance for a PV array?

It depends on the applicable standard and system type. Values of 10 ohms or less are common for lightning protection earthing, 4 ohms or less for shared grids at PV power stations in many jurisdictions, and stricter limits may apply to large substations. The project design specification is the authoritative reference.

How often should grounding resistance be tested?

At minimum, testing is required at commissioning and after any modification to the earthing system. Most O&M programs schedule annual measurements, with additional testing after severe lightning storms, flooding, excavation near the site, or observed corrosion.

Can testing be performed while the plant is energized?

Clamp-on earth resistance measurements can generally be performed on an operating plant. However, bonding continuity tests and any physical inspection of joints should be carried out with relevant circuits de-energized and locked out, following site safety procedures.

Why do measured values change after rainfall or between seasons?

Soil resistivity is strongly influenced by moisture content and temperature, so readings drop after rain and rise in dry or frozen conditions. Recording weather and soil conditions with every measurement allows seasonal trends to be interpreted correctly.

What is the difference between bonding continuity and earth resistance?

Bonding continuity confirms a low-resistance metallic path from frames and enclosures to the earthing point, measured in milliohms or low ohms. Earth resistance measures how effectively the buried electrode system dissipates current into the soil, measured in ohms. Both must pass for the grounding system to be effective.

Which instruments are required?

A calibrated earth ground resistance tester compliant with IEC 61557-5, auxiliary probes and leads, a micro-ohmmeter for continuity testing, a clamp-on ground tester for screening, and optionally a four-terminal soil resistivity meter, along with appropriate personal protective equipment.